The poll showed results for Mr Cunliffe as preferred prime minister slipped two percentage points to 11 per cent, while results for Prime Minister John Key to remain as prime minister rose three points to 51.4 per cent.

In February, 81.6 per cent of respondents to the same poll who had voted for Labour in 2011 said they would vote for them again in September.

In the latest poll this number dropped to 61.3 per cent, with the greatest drop in support affecting 30- to 44-year-olds.